Heat transfer systems such as heat pumps may be used to move heat from a source to a sink, and may underlie, for example, the operation of air-conditioning systems and/or heating systems for buildings.
Heat transfer systems can be divided into two fundamental classes distinguished by the direction in which heat moves. In one class of heat transfer system, heat flows from higher temperatures to lower temperatures. This heat flow may, for example, be harnessed to produce mechanical work, as in internal-combustion engines. A second class of heat transfer device includes systems that move heat from lower temperatures to higher temperatures. Such systems are commonly called “heat pumps.” Refrigerators and air conditioners, for example, are heat pumps.
Heat pumps necessarily consume power. In general, commonly used heat pumps employ a working fluid (gaseous or liquid) whose temperature is varied over a range extending from below that of the source to above that of the sink to which heat is pumped. The temperature of the working fluid is often varied by compression of the fluid. While conventional heat pumps may be effective in transferring or pumping heat, substantial power (in the form of mechanical work) is necessary to compress the fluid and facilitate heat transfer, making these systems relatively inefficient.